Present day communication networks must generally serve end-users requiring a broad range of integrated (data/voice/video) services. For example, a large business with hundreds of employees and numerous host machines and communication needs for facsimile, high resolution graphics and video may require up to 1 gigabit/sec. transmission capability. Other networks might operate at transmission rates in excess of 10 megabit/sec to interconnect terminals, intelligent workstations and personal computers to a large host or to interconnect small numbers of large hosts.
To achieve this capability, network operations previously performed electronically can now be implemented with optical processing, and the networks are configured with optical devices such as laser transmitters, photodiode receivers and fiber optic cables.
In many conventional electronic systems, the signals propagating over the channel facility may be expressed in terms of voltage and current waveforms. In the voltage-current domain, it is possible to generate and detect both positive and negative voltages and currents. For instance, channel signals could include, on a normalized basis, the .+-.1 signals as well as the 0 state, and detection of these signals takes advantage of the ability to sum voltages or currents to zero. With one technique, a so-called orthogonal code is utilized for encoding incoming signals and a receiver is configured as a so-called correlation detector. Because of the availability of two polarities, the overlap or projection of one pattern from the orthogonal code onto a different pattern from the code over a given time interval (basically a correlation operation) may be reduced essentially to zero. This allows for an effective detection process by the receiver since a high correlation implies that signal on the channel matches the receiver configuration whereas a low correlation indicates the signal is not destined for the particular receiver.
In an optical system, signals on a channel are propagated by the presence or absence of light energy or photons, that is, information interchange is conveyed by the ON/OFF signal states; this is in contrast to the plus, minus and zero states of electronic systems. In optics, there is no equivalent to negative values. Optical detection is equivalent to power measurement and, since power is inherently non-negative, detected signals cannot be optically manipulated to add to zero with other incident power. Thus, conventional electronic processing techniques cannot be exploited in the optical domain (or, for that matter, in any domain having a zero state and only one non-zero state), so two-state systems have required the consideration of new signal coding schemes satisfying new sets of constraints.
In a multiple user environment having only two propagation states, the considerations become even more complex. Signal separation and, ultimately, signal detection are generally achieved by time or frequency division, that is, a time or frequency slot is dedicated to each user. This is oftentimes inefficient. An alternative approach for a multiple user environment is code division. This method of transmission exploits codes in such a way that a user is able to extract the message intended for the user from the superposition of many messages on the transmission facility. Two exemplary versions of this technique are discussed in the article entitled "Coding and Decoding for Code Division Multiple User Communication Systems", published in the IEEE Transactions on Communications, Vol. COM-33, No. 4, April, 1985. Decoding in conventional code division systems is generally quite complex and a substantial portion of processing time is dedicated to decoding time, thereby reducing throughput.